I understand this part. What I am looking for an explanation of is why, if the universe were homogeneous on the largest scales, we would expect the power spectrum to be indistinguishable from zero at low ##\ell##. But it looks like you're now saying the data say something stronger--see below.

The main argument I'm making with respect to this is that you can't reasonably apply Occam's Razor to an approximation in this manner, not when we know that there's a more detailed underlying model that is more precise.

I agree that this, if true, would basically invalidate the simple homogeneous FRW model when extended much beyond our observable universe. The key point, to me, is "increasingly greater". If the power spectrum just goes flat at larger and larger distance scales, at some small variance (one part in a hundred thousand or so), that would just mean the entire universe could only be treated as homogeneous to that degree of approximation--which is basically what our current model does anyway. But if the variance increases at larger and larger distance scales, then you're right, we can't reasonably infer that any region outside our observable universe, but of comparable size, has an average density similar to our observable universe.

One thing I find interesting is that this model does predict a breakdown of the simple notion of homogeneity applied to the universe as a whole, but you can still recover it if you're willing to broaden your definition of homogeneity a bit.

In the context of cosmic inflation with a simple scalar spectral index, the primordial perturbations will, at any given time, lead to very large differences in density in different Hubble volumes. However, inflation will always end at the exact same energy density, and the subsequent expansion of the universe will continue in pretty much exactly the same way in all subsequent volumes. Thus you could say that as long as something more exotic isn't going on, the universe at 14 billion years from the local end of inflation will look pretty much exactly the same to all observers in all locations that are stationary with respect to the local Hubble flow. Depending upon the model of inflation, inflation might end everywhere within a single second, or there might be billions, trillions, or any number of years between the end of inflation in different locations (in eternal inflation models). But regardless of that timing, all of the subsequent evolution will be effectively identical, just with a different pattern of matter with the same statistical properties.

This expanded notion of homogeneity is only broken if we consider more exotic possibilities, such as spontaneous symmetry breaking leading to different low-energy laws of physics. Given that spontaneous symmetry breaking is a component of the Standard Model of particle physics, I don't think we can discount these possibilities as unlikely through the use of Occam's Razor (because you'd have to add additional assumptions to the Standard Model to discount them). If you define it so that regions with different symmetry breaking are different universes, then that's fine.